In recent years, gallium nitride (GaN) based compound semiconductor light emitting device has been attracting attention as a short wavelength light emitting device. This gallium nitride based compound semiconductor light emitting device is formed using a method such the metal-organic chemical vapor deposition method (MOCVD method) or the molecular beam epitaxial method (MBE method) on substrates of various oxides or group III to group V compounds starting with sapphire single crystal.
Small electric current diffusion in the transverse direction is one of the features of gallium nitride based compound semiconductor light emitting device. For this reason, current is injected only in the semiconductor directly below the electrode, the light emitted from the light emitting layer directly below the electrode is blocked off by the electrode, and it becomes difficult to extract the light from the light emitting device. That is why such light emitting devices are configured by generally using a transparent electrode as the positive electrode so that light can be extracted through the positive electrode.
Conventional light-permeable positive electrodes made of transparent electrodes were stacked structures made by combining oxides of Ni, Co and the like with a contact metal such as Au. In recent years however, higher electrically conductive oxides, such as ITO (In2O3—SnO2) are being used, the film thickness of contact metal has reduced considerably, positive electrodes with stacked structure of light-permeability have been adopted, and techniques for extracting light with a high efficiency from the light emitting layer are being proposed.
The external quantum efficiency of such light emitting devices, however, is expressed as the product of the light extracting efficiency and the internal quantum efficiency. The internal quantum efficiency is the part of the current energy injected in the light emitting device that has been converted to light. On the other hand, the light extracting efficiency is the part of the light generated within the semiconductor crystal that can be extracted outside it.
In recent years, the internal quantum efficiency of the light emitting device is said to have improved by as much as 70% to 80% by improvements to the crystalline state and studies on the structure so that adequate effects corresponding to the injected current quantity are now being obtained.
However, the light extracting efficiency corresponding to the injected current is generally low not only in gallium nitride based compound semiconductor light emitting devices but also in light emitting diodes (LED), and the internal emitted light is not being extracted outside adequately compared to the injected current.
The emitted light extracting efficiency in gallium nitride based compound semiconductor light emitting devices is low because the light emitted in the light emitting layer is repeatedly reflected and absorbed by the crystalline material in the light emitting device structure. When reflection exceeds the critical angle according to Snell's Law, the probability that light is not extracted from outside the light emitting device, increases.
To improve the light extracting efficiency further, a gallium nitride based compound semiconductor light emitting device with improved light extracting efficiency obtained by roughening the emitted light extracting surface and by setting various angles to the light extracting surface has been proposed (for instance, see Patent Document 1).
However, the gallium nitride based compound semiconductor light emitting device produced by the method stated in Patent Document 1 has the problem that interference effect occurs in the uneven pattern on the semiconductor surface formed by the mask patterning method, and only light of a specific wavelength is emphasized.
Moreover, since a fine mask patterning method is required in the method of roughening the semiconductor layer surface, a complex and troublesome procedure has to be adopted in the processes, and the production efficiency drops.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H6-291368.